1. Field of the Invention
This invention relates to a method and apparatus for controlling the fuel injection of an internal combustion engine and more particularly to a method and apparatus for changing over between different fuel injection modes in accordance with an engine operating condition.
2. Description of the Prior Art
With the conventional electronic fuel injection systems of the type including fuel injectors, a method has been used in which the injection timing of each cylinder is controlled independently from other cylinders to improve the acceleration response characteristic and to increase the degree of freedom of fuel metering at low engine speeds (this mode of injection is hereinafter referred to as an independent injection).
Also, Japanese Patent Publication No. 49-45652 (1974), for example, discloses a fuel injection control in which the independent injection mode is changed over to a simultaneous injection mode to inject the fuel to all cylinders simultaneously, since it is difficult at high engine speeds to control the fuel injection in the independent injection mode by the use of an inexpensive microcomputer having a low computing speed. However, the use of this change-over method always supplies the fuel excessively and insufficiently to some cylinders during each change-over period as shown in FIG. 1 so that the combustion is deteriorated and the drivability and exhaust emissions are affected adversely. This phenomenon will be explained with reference to FIG. 1.
FIG. 1 is a timing chart showing the times (b in the Figure) during which the intake valves open and the injection pulse signals (a, a' in the Figure) generated upon change-over from the independent injection mode to the per revolution simultaneous injection mode. If a change-over from the independent injection mode to the simultaneous injection mode and a change-over from the simultaneous injection mode to the independent mode are effected at crank angles 5/3 .pi. and 29/3 .pi., respectively, as shown in the Figure, the ratios of the fuel amounts drawn into the respective cylinders to the engine requirements become as shown in the Figure so that each of the Nos. 2, 4 and 1 cylinders has an excess or deficiency in fuel of about 50% and the engine will misfire during the three combustion periods. Indicated at c in the Figure are the ignition signals. If, for example, this control is effected by an inexpensive microcomputer having a low computing speed, there is a serious problem in that the times of change-over between the independent injection and the simultaneous injection are limited to the low engine speed range in view of the computing capability and the drivability and exhaust emissions are deteriorated.